IQ modulators are well known in the field of RF and microwave communications, finding use in both analog and digital modulation formats. IQ modulation is a method of modulating a carrier wave, which is typically but not always sinusoidal, with two baseband input signals. The two signals are oftentimes referred to as I (in-channel) and Q (quadrature-phase) components.
FIG. 1 is a block diagram of an example conventional I-Q modulator 5. It contains a local oscillator (or “LO”) 10 producing sinusoidal signals at a carrier frequency, designated as ωc. The LO 10 has two outputs, which are of equal magnitude and differ in phase by exactly 90 degrees. The signal from the LO 10 are multiplied in mixers 12, 14 by two independent baseband inputs, the I and Q inputs. These products of the I and Q inputs and the carrier frequency ωc are summed to yield the frequency-converted result. Baseband inputs may contain any arbitrary waveforms, although the bandwidth of these is usually less than the carrier frequency.
In FIG. 1, the baseband inputs are designated as x (in-phase) and y (quadrature), while the two LO signals are designated I and Q. When represented using phasor notation at the carrier frequency, the two LO signals are simply:I=ej0=1,Q=ejπ/2=j 
The output of the modulator 5 is the sum of these two quadrature LO signals I and Q multiplied by the two baseband modulation inputs (represented by {x,y}):z=xI+yQ=x+jy 
In this way, the I-Q modulator 5 up-converts the real-valued baseband inputs {x, y} as if taken together, they were together a complex-valued input (x+jy).
FIG. 2 is a phasor diagram showing an ideal I vector 20 and an ideal Q vector 22. Although ideal modulators will generate I and Q channels that have exactly the same amplitude gain across the desired frequencies, and will be out of phase from one another by exactly 90 degrees, real-world implementations of the I and Q signals do not have identical magnitudes and do not differ in phase by exactly 90 degrees. Additional non-ideal aspects of the I-Q modulator, such as differing gains and phases between the two mixers, can also be modeled as amplitude and phase imbalances between the I and Q LO signals. These imbalances may affect the quality of the generated signal from the modulator.
Without loss of generality, the I vector can be arbitrarily defined to be one, and then the Q vector may be restated as:Q=(1+ε)ej[π/2+γ],
where ε and γ represent errors in magnitude and phase respectively. This error itself may be represented in the phasor diagram. For example, in FIG. 2, this errant Q vector is depicted by the vector 24. Such errors are also referred to as “IQ imbalance,” and often vary with changes in operating parameters, such as carrier frequency and drive power of the local oscillator 10.
Imperfections in the design and construction of IQ modulators result in apparent DC offsets being present at the {x, y} inputs. This causes problems with the modulated signal at exactly the carrier frequency. Often it is desirable to remove the signal component at the carrier frequency. To achieve this result in a modulated signal, the analog waveforms presented at the {x, y} inputs would normally be designed to have a zero average DC level. Unfortunately, DC offsets in the modulator result in an undesired signal component at the carrier frequency under these conditions.
It is therefore important to adjust the analog input waveforms' average DC level to compensate for internal offsets in the IQ modulator, thus removing the carrier component from the modulated signal. Similar problems occur when a component with specific magnitude and phase must be intentionally generated at the carrier frequency.
It is not possible to remove the carrier component from the IQ modulator output without determining the DC offset, which must first be measured. One conventional method for measuring DC offset is illustrated in FIG. 3. In a typical method, the DC average of the x input is adjusted to minimize the carrier signal amplitude. Generally this will not reduce the carrier amplitude to zero because of the unknown DC offset present at the y input of the modulator. Thus, the standard way to measure offset becomes an iterative process, where first DC average of the x input is first adjusted, then the y input is adjusted. Adjusting the y input, however, affects the x offset, so then the x input is adjusted again, followed by the y input being adjusted again. Each iteration typically reduces the amount of adjustment made in subsequent iterations until finally the carrier amplitude is driven to acceptably low levels. An additional complication is that the variation of carrier power versus DC offset is not linear, and present techniques use various iterative search techniques to locate an offset that minimizes carrier power. FIG. 3 illustrates only the first four iterations, but in practice the final DC measurement may involve many iterations.
As may be imagined from the above description, conventional measurement techniques for measuring DC offset such as the iterative technique described above suffer from deficiencies such as being tedious and time consuming to measure. Thus conventional DC offset testing methods are inaccurate, take too long, or both.
Embodiments of the invention address these and other limitations of the prior art.